Size of action potential is decreased as a result of: March 2010
**Core Concept**
The size of an action potential is determined by the difference in resting and threshold membrane potentials, which is influenced by the balance of sodium (Na+) and potassium (K+) ions across the neuronal membrane. The action potential is initiated by an influx of Na+ ions through voltage-gated sodium channels, leading to a rapid depolarization.
**Why the Correct Answer is Right**
A decrease in the size of an action potential can be caused by a reduction in the number or function of voltage-gated sodium channels. This can be due to various factors such as hypokalemia (low potassium levels), hypercalcemia (elevated calcium levels), or the presence of certain drugs that block sodium channels. For example, in the case of hypokalemia, the reduced potassium concentration leads to a decrease in the outward potassium current, making it more difficult for the membrane to repolarize, and thus, reducing the size of the action potential.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided, so we will proceed with the remaining options.
**Option B:** This option is not relevant to the question, as it does not directly affect the size of the action potential.
**Option C:** This option is incorrect, as an increase in the concentration of sodium ions would actually increase the size of the action potential, not decrease it.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that changes in the size of action potentials can have significant effects on neuronal function and excitability, and can be a critical factor in the development of various neurological disorders.
**Correct Answer:** C. Hypercalcemia.